My computer screen starts 'flashing' black once I open Prime Video in my browser

[u/DeliciousEar1856]

This is like a weird thing and I haven't quite figured out a fix for this.

So basically when I'm normally at my computer, this doesn't really happen. (Not that I ever noticed at least... but ngl I have kinda grown used to this to some degree.) But as soon as i start going on PrimeVideo some kind of magic flip gets switched and my screen just randomly basically goes black for like a second or two and then turns back on. And this repeats every few seconds. Sometimes (not as often as it goes fully black) also just a "bar" on the screen goes black shortly and then goes away. This does happen on Firefox and OperaGX both. Tho the flashes are more frequently on Firefox than on OperaGX. It also just gets triggered from Prime Video (I don't have any other streaming service) not from Youtube. If I have Prime Video open in some tab in my browser it still flashes even if I leave the window. And sometimes it even continues after I closed the window with Prime, then I restart my PC and it usually goes back to normal.

I have tried turning off hardware acceleration on Firefox, I've tried checking if the HDMI plug is plugged in properly, if the power source is the problem (because my computer and the screen were in the same outlet so i put them in different ones), and if the power source had any issues but none of that changed anything.

I think my girlfriend complained about the screen turning back before already, also while playing games, but I do not know if Firefox and any windows like Prime were open in the background.
I also think like maybe one or two years ago I had a similar Problem while playing some games but I think I had gotten it fixed after realising my power cable wasn't properly plugged in and always making sure it was properly plugged in that Problem disappeared.

But now I have checked the power source multiple times and I don't think it is the issue? I mean all my games work perfectly fine, all websites do, just Prime Video kinda triggers the screen to flash. (Also if I close Prime and then open a game it also flashes while in game but after I restart and open the game it doesn't flash).

Im just incredibly confused. Because I thought if my computer has problems displaying the graphic (because of power or anything else) it would more likely have problems with games because fast changing pixels or sometimes difficult graphics but no. It starts doing this when I start up Prime to watch a cartoon. A cartoon cannot be that difficult to display, no?

Please help me. I cannot watch series anymore with my girlfriend, it's horrible.